1. Field of the Invention
The present invention relates to an optical circulator used for optical communication and, more particularly, to an optical waveguide circulator.
2. Description of the Related Art
The optical circulator is an optical device having at least three optical input/output ports A, B, and C and provided with a function that, for instance, light incident on port A is allowed to exit from port B and light incident on port B is allowed to exit from port C, but light is prohibited from passing through the device in the opposite direction.
Conventionally, as shown in FIGS. 1 and 2, an optical circulator is generally constructed by bulk components such as polarizing beam splitters 12, Faraday rotators 10, wave plates 11, and prisms 13.
As shown in FIG. 1, light inputted to port 14 (port B) is separated by the polarizing beam splitter 12, and resulting beams are rotated in polarization plane by 45.degree. in passing through the Faraday rotators 10. Resulting 45.degree.-rotated beams are rotated by 45.degree. in the opposite direction by the wave plates 11, then combined by the other polarizing beam splitter 12, and outputted from port 15 (port C).
On the other hand, as shown in FIG. 2, light inputted to port 16 (port A) is separated by the polarizing beam splitter 12, and separated beams are rotated in polarization plane by 45.degree. in passing through the wave plates 11. Resulting 45.degree.-rotated beams are further rotated by 45.degree. in the same direction by the Faraday rotators 10, then combined by the other polarizing beam splitter 12, and outputted from port 14 (port B).
In the above bulk-type optical circulator, each of the polarizing beam splitters 12, the Faraday rotators 10, and the wave plates 11 is made of quartz glass or a magnet, for instance. In assembling those optical elements, to efficiently guide incident light to an output end, care should be taken not to cause deviations from the optical axis.
FIG. 3 shows another optical circulator which is configured differently from the above one. That is, the optical circulator is configured by two optical isolators and one Y branching device connected to the former.
Constructed as an assembly of a plurality of optical elements, the conventional bulk-type optical circulator necessarily has a limit in its miniaturization. In addition to limitations due to sizes of the optical elements themselves, there are other factors of preventing the miniaturization as exemplified by the facts that such operations as polishing and bonding of glass members are needed to produce a polarizing beam splitter, and that optical elements need to be assembled with each other.
The optical circulator of FIG. 3 also has a limit in miniaturization because the individual parts, i.e., the isolators and the Y branching deice, are large and a certain space is needed for reinforcement after the parts are connected together.
The conventional optical circulators have another problem that the individual optical elements are expensive. For example, each of such optical elements as a polarizing beam splitter, a wave plate, and a Faraday rotator is expensive and hence prevents price reduction. Also in the configuration with fusion splice of the parts, the parts, i.e., the isolators and the Y branching device, are expensive and the cost of fusion splice is high.
Further, requiring assembly of optical parts with high accuracy, the conventional optical circulators has a limit in mass productivity even with automated assembly using robots or the like. Also in the configuration with fusion splice of the parts, the parts are connected together such that optical fibers are fusion-spliced manually one by one and hence there is no likelihood of substantial improvement in mass productivity.
Still further, when a conventional circulator is used in combination with a passive device, it is unavoidable to manually fusion-splice optical fibers of the optical circulator and the passive device one by one and hence the manufacture takes long time.